1. Field of the Invention
The present invention relates to a can manufacturing apparatus, a can manufacturing method, and a can, wherein the apparatus for manufacturing the can has a dome section and an annular projection, the dome section being formed on a bottom of the can and is recessed inwardly of a body of the can, and the annular projection being formed around the peripheral edge of the dome section so as to project outwardly in a can axis direction.
2. Discussion of Background
Two-piece aluminum alloy cans composed of a cylindrical bottomed can body is well known. A can end is fixed to an opening in the can body. The can body of the two-piece can is made by, first, stamping and drawing an aluminum alloy blank sheet to form a cup member using a drawing apparatus. Then, the cup member is redrawn and ironed with a punch sleeve inserted therein, while the cup member is held. In this way, the peripheral edge of the bottom portion of the cup member is drawn, while it is clamped between the punch sleeve and a dome molding unit. The dome molding unit has a semi-spherical extreme end and is disposed in opposition to the punch sleeve so as to be coaxially therewith. Thus, a can body 101, having the bottom shape shown in FIG. 13, is obtained.
The can bottom 103 of the can body 101 has a dome section 105 and an annular projection (rim) 107 formed thereon. The dome section 105 is spherically recessed inwardly of the can body 101, and the annular projection (rim) 107 joins the peripheral edge of the dome section 105 and projects outwardly of the can body 101 in a direction of the can axis. The annular projection 107 acts as a leg, which is in contact with the ground, when the can body 101 stands upright. Thus, the standing stability and supporting strength of the can body 101 can be improved.
Today's trend toward reduced wall thickness of cans to save resources and costs has lead to various disadvantages due to the reduction of the strength of the cans. One disadvantages results from a phenomenon called bottom growth, wherein an annular projection 107 of a can bottom 103 is deformed outwardly in a radial direction, while projecting downwardly due to the action of internal pressure, after contents are packed into the can (see the double dot and dashed line of FIG. 14). One of factors which causes bottom growth is the insufficient rigidity of an inner wall 107a, acting as the inner peripheral wall of the annular projection 107. A first peripheral edge of the inner wall 107a is joined to the dome section 105 through a counter sink R-section 109 to form a concave surface. A second peripheral edge of the inner wall 107a is joined to a nose section 107b to form an extreme end of the annular projection 107. When an internal pressure acts on the can bottom 103, the thin inner wall 107a is stressed in the circumferential direction and in the direction of the can axis. In particular, when the stress (elongation) in the direction of the can axis is increased, the annular projection 107 is deformed downwardly and radially outwardly.
When bottom growth occurs, the total height of a can is increased. Thus, problems may arise due to the can's increased height, such as the cans may become caught on a conveyor as the can is transported to be packaged or the cans may be difficult to package.
Another problem is that as the thickness of the can is reduced, the falling strength of a can bottom becomes insufficient. In other words, when a can falls, the peripheral portion of the dome section 55 of the can bottom 53, which is a particularly fragile portion when subjected to shock caused by a can's fall, swells and may be broken in the worst-case scenario.
In view of the above-described disadvantages, an object of the present invention is to provide a can, a can manufacturing apparatus and a can manufacturing method, wherein the apparatus and method are for manufacturing a can having a reduced wall thickness, particularly in the can bottom, yet retaining sufficient can strength so as to prevent deformation of the can bottom and suppress bottom growth.